Field of the Invention
The present invention relates to thermoplastic compositions comprising vinylaromatic (rubber-modified) copolymers prepared by the emulsion polymerization process and precipitated with an alkaline earth metal salt, preferably with magnesium sulfate, which as a result of the preparation comprise residues or the total amount of the salt used for the precipitation, wherein the moldings prepared from the compositions are characterized by an improved surface quality after storage under warm humid conditions. The moldings preferably have a class A surface, free of visual defects and stable to aging under these conditions.
Description of Related Art
The present invention also relates to a method for preparing compositions according to the invention, the use of the inventive compositions for preparing moldings and also moldings which were prepared from the inventive compositions.
The compositions comprising vinylaromatic (rubber-modified) copolymers prepared by the emulsion polymerization process, which as a result of the preparation contain salt inclusions, are known from the literature. Sources of such salt inclusions as a result of the preparation are manifold, for example, emulsifying solutions, polymerization initiator solutions, buffer solutions and particularly precipitating solutions used in the emulsion polymerization process as auxiliaries, which, depending on the method, remain in the material or are only incompletely removed again from the material during the work-up of the polymer.
Particularly in traditional preparation methods, the precipitation of vinyl polymer lattices, by means of addition of acids and/or salts, prepared by emulsion polymerization described, for example, in EP 459 161 B1, DE 2 021 398 and DE 28 15 098, contribute to the salt burden of the final polymer to a considerable degree, since removal of these salts again from the product via downstream method steps (washing) is generally possible only to an inadequate degree and/or at high cost (energy and water/waste water). The coagulants used are, for example, and preferably, aqueous solutions of water-soluble salts such as alkali metal, alkaline earth metal or aluminum chlorides, sulfates, nitrates, phosphates, acetates, formates, aluminates or carbonates, particularly preferably aluminum chloride, calcium chloride and magnesium sulfate solutions, optionally in combination with inorganic or organic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, formic acid, acetic acid, propionic acid and citric acid.
It is described in the literature that such salt inclusions in compositions comprising vinylaromatic rubber-modified copolymers can lead to undesired effects. This is known in particular for compositions comprising vinylaromatic copolymers in which the vinylaromatic copolymer has been precipitated with an alkaline earth metal salt, preferably selected from the group of magnesium and/or calcium sulfates or chlorides, particularly preferably selected from magnesium sulfate and calcium chloride, especially preferably magnesium sulfate, in basic media, i.e. preferably in the absence of an inorganic or organic acid.
Base-precipitated vinylaromatic rubber-modified copolymers, in contrast to those precipitated with acid, have the advantage that these contribute less markedly to hydrolysis and processing instability during processing in a polycarbonate blend using acidic stabilizers (cf. EP 2373743).
By way of example, WO 2009/071537 discloses that magnesium compounds and/or calcium compounds in impact-modified vinylaromatic copolymers selected from the group of the acrylonitrile-butadiene-styrene copolymers (ABS), acrylonitrile-styrene-acrylate-copolymers (ASA) and methacrylate-acrylonitrile-butadiene-styrene copolymers (MABS), optionally comprising polycarbonate and additives, lead to undesired deposit formation on the shaping mold during thermoplastic shaping via injection molding or extrusion, and to that extent claims compositions of this type having a content of magnesium compounds and/or calcium compounds of 0 mg/kg to 100 mg/kg. The emulsion polymers used in said compositions are usually precipitated by freeze precipitation in a flake-ice machine, rather than by the conventional addition of magnesium sulfate solution.
WO 98/28344 discloses a method for the continuous coagulation of aqueous dispersions of graft rubbers via shear, which overcomes the known disadvantage of precipitation by means of acids and/or salts as coagulants, that contaminants often remain in the work-up polymers, and these can lead to impairment of product properties.
One problem with thermoplastic compositions comprising vinylaromatic copolymers prepared by the emulsion polymerization process with salt inclusions resulting from the preparation is that when moldings produced from these are exposed to moisture (for example condensation or humidity), particularly at elevated temperatures, they are susceptible to undesired development of surface defects (blistering), which restrict the use of compositions of this type in moldings with high-gloss finish and Class A surface requirement.